What I learned about Windows and Daylight Saving Time today...

Prompted by a bug report regarding the new DST rules I had a closer
look how localtime() determines DST on Windows.  Below is my analysis
of the sorry state of affairs, as far as I can tell.

I guess for the vast majority of Windows users this just means that
DST will be wrong for older timestamps.  I'm not sure how important
that is though.  Providing a proper workaround probably means that
we would have to use the DateTime::TimeZone module within localtime().

=====================================================================
The daylight saving time (DST) for the localtime() function is
determined by the C runtime library function of the same name.

This function has 2 different operating modes:

1) The TZ environment variable has been set.

In this case MSVCRT calculates DST itself, based on the USA rules.
The latest released version (as of (March 5th 2007) of MSVCRT.dll has
not yet been updated for the 2007 DST rules.  The localtime() function
will still apply the old rules to 2007.  Microsoft claims to have a
hotfix for this, that is available from Microsoft Customer Support
Services only:

   http://support.microsoft.com/kb/932590

2) The TZ environment variable has *not* been set.

In this case MSVCRT retrieves the DST transition times from the
GetTimeZoneInformation() API:

    http://msdn2.microsoft.com/en-us/library/ms724421.aspx

This API only provides the transition times according to the *current*
DST rules.  There is no database of historical transition times.  That
means that localtime() applied to previous years will use the new
transition times even for old timestamps.

Windows Vista (and Longhorn Server) support a new API called
GetDynamicTimeZoneInformation() that implements a database of historic
DST rules.  It is not known if the hotfix mentioned above will make
use of this API when running on Vista.
=====================================================================

Cheers,
-Jan

• What I learned about Windows and Daylight Saving Time today... by Jan Dubois
• Re: What I learned about Windows and Daylight Saving Time today... by Dominic Dunlop
• RE: What I learned about Windows and Daylight Saving Time today... by Jan Dubois